1. Field of the Invention
The present invention relates to an optical wavelength demultiplexer which separates light that contains a plurality of wavelengths into individual components in accordance with their respective wavelengths.
2. Description of the Prior Art
In order to increase transmission capacity in optical communications it is general practice to use an optical wavelength multiplexing means in which a plurality of optical signals of different wavelengths are propagated over the same optical transmission line. In optical communications employing this type of optical wavelength multiplexing means, it is necessary to provide an optical wavelength demultiplexer on the receiving side of the optical transmission line for the purpose of separating the multiplexed optical signals in accordance with their respective wavelengths.
As optical wavelength demultiplexers employed for the above-described purpose, various types of elements have heretofore been proposed. These optical wavelength demultiplexers generally fall into three types: angular dispersive elements which use diffraction gratings or prisms; wavelength selective transmitting or reflecting elements which use interference filter films; and optical waveguide circuits.
FIG. 1 shows the arrangement of a conventionally proposed optical wavelength demultiplexer of the type which employs interference filter films. In this optical wavelength demultiplexer, signal light consisting of light of a wavelength .lambda..sub.1 and light of a wavelength .lambda..sub.2 is input from an input fiber 11, and the signal light is separated into the light of the wavelength .lambda..sub.1 and the light of the wavelength .lambda..sub.2, which are respectively output to output fibers 12 and 13.
Between the input and output fibers 11, 12 are disposed lenses 15-1 and 15-2 which interpose a filter film 14-1 therebetween. The end portion of the input fiber 11 is optically attached to one end side of the incident-light surface of the lens 15-1, so that the light of the wavelength .lambda..sub.1 in the signal light refracted by the lens 15-1 passes through the filter film 14-1, whereby the light of the wavelength .lambda..sub.1 selected by the filter film 14-1 is refracted at one end side of the emergent-light surface of the lens 15-2 and is taken out by means of the output fiber 12. The light of the wavelength .lambda..sub.2 in the signal light is reflected by the end surface of the lens 15-1 and enters a lens 15-3 which is optically brought into contact with the other end side of the incident-light surface of the lens 15-1. The light of the wavelength .lambda..sub.2 entering the lens 15-3 passes through a filter film 14-2 which selectively transmits the light of the wavelength .lambda..sub.2 and is refracted at one end side of the emergent-light surface of a lens 15-4 and is taken out by means of the output fiber 13.
In the conventionally proposed optical wavelength demultiplexer of the type described above, such as that illustrated in FIG. 1, the wavelength demultiplexer portion is provided independently of the optical fibers and, therefore, the structure as a whole is disadvantageously complicated and increased in size. At the same time, it is inconveniently necessary to accurately effect complicated operations, such as a fine adjustment of optical axes, in the manufacturing process. Even if such adjusting operations have been properly effected, the adjusted operating conditions may be unfavorably changed by external disturbances, such as temperature changes or externally applied vibrations. Accordingly, it is not possible to maintain a stable operation over a long period of time without the need for a readjusting operation.